How deep should my propeller be?

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Spawn Sack

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I have a 40Lb trust Min Kota electric on my 12 foot aluminum v-hull. I've been playing around with how deep the propeller is trying to find out which depth is "best." I suppouse "best" to me would be the least amount of drag thus allowing the boat to cruise as fast as possible. I'm also guessing that when trolling at low speeds, if there is less drag in the propeller area then I would have to use less throttle to achieve the same speed, thus saving some life on my battery.

Anyway, I've done some reading online and read some sites that say something to the effect of "just deep enough so that the prop does not cause bubbles to form on the surface." I don't find this terribly useful as at low speeds a certain depth may not cause bubbles, and at more throttle it may cause bubbles.

Another site said to have the center of the prop 9 inches below the surface of the water. Alright, but it's a bit hard to measure this while the boat and me are in the water. I suppouse I could mark the water line with tape, then when I get out on shore measure 9 inches down from that.

My fish finder does not have speed otherwise I could play with the depth and figure out what works best.

Obviously the boat will not be moving that fast no matter what depth the propeller is at. However when nature calls minutes do count when putting towards shore (!) And if I could save a bit of juice on my battery that would be nice too.
 
On a transom mount, you want the prop to be below the hull. On a bow mount, you want it deep enough that it doesn't cavitate when you move around the boat.
 
[url=https://www.tinboats.net/forum/viewtopic.php?p=322614#p322614 said:
krawler » Yesterday, 20:50[/url]"]On a transom mount, you want the prop to be below the hull. On a bow mount, you want it deep enough that it doesn't cavitate when you move around the boat.
Winner winner chicken dinner! You don't want to prop to hit the boat and cause damage (mainly to the prop) nor do you want it to cavitate. I like to keep it as high as posible and achive this. Not so much for less drag, but for when I go through shallow water or an area with a lot of grass or submerged structure.
 
[url=https://www.tinboats.net/forum/viewtopic.php?p=322594#p322594 said:
Colbyt » 16 Jul 2013, 16:55[/url]"]I think your manual says about 12" below the bottom of the boat.

Believe it or not it doesn't specify. I read it over very closely.

Make's sense about having it below the hull so there is no chance of the prop hitting the boat. When the boat is on the trailer I'm going to lower the prop so it is just below the hull and make sure there is no contact if turn it 360 degrees.

I'll try it at this depth first and if there is no cavitation I'll likely leave it there as, like krawler pointed out, there is less chance of having weeds etc get caught up in the prop when going through shallow water.
 

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